The effects of cryosurgery on the ureter and kidney were evaluated in dogs. Repeatedly frozen ureters developed partial obstruction within two weeks. The majority of animals progressed to stenosis and/or ureteral obstruction on long term followup. Often dogs were subjected to in situ cryoinjury to the kidney while renal-vascular changes were monitored by selective arteriography. Vascular flow ceased at temperatures of minus 30 degrees C and visceral microscopic changes were observed in the cryolesion in excess of minus 50 degrees C. A predictable and sharply defined lesion was thereby produced in normal renal parenchyma.